To find out why Christians are against pre-marital (and extra-marital) sex, listen to Pastor Robert McClury's great sermon on Matthew 5:27,28:
Dealing With Lust, by Pastor Robert McCurley
The Bible says:
"Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
And
if thy right hand offend thee, cut it off, and cast it from thee: for
it is profitable for thee that one of thy members should perish, and not
that thy whole body should be cast into hell." - Matthew 5:2-30
Recommended reading:
Why Christians Make Miserable Addicts - by Heather Kopp/huffingtonpost.com
Women Want Real Love, Not Fifty Shades Of Counterfeit - wnd.com/internetsafety101.org
Women Are From Venus and Men Are From Whatever Planet Watches Porn All the Time - my post